The present invention relates to an improved signal processing circuit for an encoder, and more particularly relates to improvement in resolution of a signal processing circuit used for a magnetic or photoelectric encoder for detection of an angular or linear displacement of a mobile object.
A number of signal processing circuits have already been developed in the field of encoders used for the above-described purposes. One of the most typical conventional signal processing circuit is described in Japanese Patent Application Sho. No. 61-54288. This signal processing circuit is adapted for use in a magnetic rotary encoder. A scale is formed by a magnetic disc having a track formed on its periphery and the track is magnetized with a sine wave having a constant period. The wave length .lambda. of the sine wave used for the magnetization is preferably chosen in a range from several tens to several hundreds .mu.m.
A pair of magnetic sensors are arranged in spaced relation to and facing the track on the scale to generate level signals corresponding to the intensity of magnetization of the scale. These magnetic sensors are made of a device which includes no carrier waves in its output signals. Most typically, a magnetic wafer or a semiconductor element is preferably used for the magnetic sensors. The magnetic sensors are arranged so that their output signals have a phase difference of 1/4.lambda. (90.degree.). In other words, the magnetic sensors are spaced apart from each other by a distance equal to (i.+-.1/4.lambda.) in the longitudinal direction of the scale, when i is a positive integer. The magnetic sensors and the scale are arranged for relative displacement in a manner such that the magnetic sensors move around a fixed scale or the magnetic sensors are fixed about a rotatable scale.
As is clear from the foregoing, the second magnetic sensor issues a cosine wave signal when the first magnetic sensor issues a sine wave signal. As a consequence, when the length of one period of the sine wave used for magnetization is set to .theta.=0 to 2.pi., the magnetic sensors issue analog signals sin .theta. and cos .theta..
The magnetic sensors are connected to respective A/D converters which convert the analog signals sin .theta. and cos .theta. to corresponding digital signals. The digitalized signals sin .theta. and cos .theta. are then applied to one input terminal of respective digital multipliers. The output signals of the multipliers are supplied to input terminals of a digital reducer whose output signal is applied to a digital comparator.
The comparator issues an up/down signal (U/D) which is applied to the up-down switch terminal U/D of the first counter. Here, the U/D signal assumes a value "1" when the result of reduction at the reducer exceeds 0 and a value "0" when the result does not exceeds 0. The counter counts clock signals CK periodically inputted thereto in a known manner. The counter operates in an up-count mode on receipt of a "1" signal and in a down-count mode on receipt of a "0" signal at its up-down switch terminal U/D.
A function generating ROM is interposed between the counter and the multipliers. This function generating ROM issues sine and cosine data corresponding to a count value .phi. at the counter. That is to say, data sin .phi. and cos .phi. data are prestored in the function generating ROM and, depending on the count value .phi. received from the counter, that data is sequentially read out. The data sin .phi. and cos .phi. data are supplied to the other input terminal of the digital multipliers.
With such a construction of the conventional signal processing circuit, one digital multiplier issues a signal sin .theta..multidot.cos.phi. whereas the other multiplier issues a signal cos .theta..multidot.sin.phi.. As a result, the digital reducer issues a signal sin .theta..multidot.cos.phi.-cos .theta..multidot.sin .phi.=sin (.theta.-.phi.). On receipt of this output signal, the digital comparator makes the U/D signal equal to "1" for the positive value of the output signal and to "0" for the negative value of the output signal. The count value .phi. of the counter varies depending on the polarity of the signal sin (.theta.-.phi.). The digital multipliers and the function generating ROM in combination form a function generating unit. However, the comparator and the counter form in combination a counting unit.
The magnetic sensors are also connected to the first and second wave shaping circuits each of which discriminates the output signal from the associated magnetic sensor by means of a threshold value to convert into a 2 value signal of a "1" level and a "0" level. Output signals from the wave shaping circuits take the form of square waves which are out of phase relative to each other by .pi./2. When the relative displacement between the scale and the magnetic sensors is in a positive direction, the output signal from the first wave shaping circuit precedes in phase and, when negative, the output signal from the second wave shaping circuit precedes in phase.
The wave shaping circuits are connected to a common direction discriminator which detects the polarity of the relative displacement. Such discrimination is carried out, for example, depending on the level of the output signal from the first wave shaping circuit at the rising of the output signal from the second wave shaping circuit. The output signal from the direction discriminator is applied to the up-down terminal of a counter and to the outside of the system.
The counting mode of the counter is effected by the output signal from the direction discriminator and, at the same time, counts the pulses from one wave shaping circuit. The counter is operated in to an up-count mode for the positive direction of the above-described relative displacement and in a down-count mode for the negative direction of the relative displacement. Once every relative rotation between the magnetic sensors and the scale, a 0-point signal is issued at the reference position of the scale. After passing through the third wave shaping circuit, the 0-point signal is converted into a 0-point pulse which is then passed to the reset terminal of the second counter. As a result, the second counter is reset every time the magnetic sensor pass by the reference position on the scale. As a consequence, the count value of the second counter corresponds to the number of magnetic domains, which is equal to the number of magnetic poles, passed by the magnetic sensors in the area between their current position and the reference position. The first to third wave shaping circuits, the direction discriminator and the second counter thus form a scale domain counting unit.
With the above-described construction and function, the output signal N from the second counter forms the higher bit data of the displacement data Dout whereas the count value .phi. of the first counter from the lower bit data of the displacement data Dout.
As is clear from the foregoing description, the second counter counts up every time the magnetic sensors pass by one magnetic domain at the positive displacement between the magnetic sensors and the scale and the count values are supplied to the higher bits of the displacement data Dout. Thus the higher bits of the displacement data Dout indicates the number of the magnetic domains passed by the magnetic sensors, i.e. the current position of the magnetic sensors with respect to the reference position on the scale.
Further, after conversion by the A/D converters, the analog signals sin .theta. and cos .theta. from the magnetic sensors are respectively multiplied with the signals cos .phi. and sin .phi. issued by the function generating ROM. The result of multiplication is processed through the reducer to produce a signal sin (.theta.-.phi.). Depending upon the polarity of this signal, the comparator makes the U/D signal equal to "1" or "0" to vary the count value .phi. of the first counter. Depending upon the count value .phi., the output signal sin .phi. and cos .phi. from the function generating ROM vary. As a consequence, a sort of digital phase locked loop is formed by the multipliers, the reducer, the comparator, the first counter, the function generating ROM and the multipliers.
The above-described digital phase locked loop operates so that the value of sin (.theta.-.phi.) should always be equal to 0, i.e. .theta. should always be equal to .phi.. Thus the count value .phi. of the first counter indicates the position of the magnetic sensors within a certain magnetic domain. When the first counter is provided with 8 to 10 bits, the value .theta. in a range form 0 to 2.pi. is divided into 256 to 2048 fractions. In other words, the degree of resolution in detection by the count value .phi. has a precision of 1/256 to 1/2048 of a magnetic domain.
In the case of signal processing by the above-described signal processing circuit, the analog signals sin .theta. and cos .theta. issued by the magnetic sensors are first converted into corresponding digital signals, and the common value .theta. is then divided finely by the digital phase locked loop and the divided data, i.e. the count values .phi. are issued as the displacement data. In order to raise the resolution in detection under this condition, one possible expedient is to increase the number of bits at the first counter to raise the dividing function of the digital phase locked loop. For various economic and technological reasons, however, there is a certain limit to increasing the bit number at the first counter.
Since the displacement data are given in the form of digital signals, transfer of the data to an analog control circuit or the like requires use of a D/A converter, which inevitably raises installation costs and increases processing time.